It does not make sense to show the middle seat blocked while the adjacent aisle and window seats are shown empty. This further encourages passengers to book aisle and window which is undesirable from the airline's perspective.

But someone really concerned about carry on space may book the middle seat, since there is a greater chance the aisle or window seats may have smaller under seat storage due to placement of the seat legs or struts.

Travel tips:
http://members.aol.com/ajaynejr/travel.htm